Display devices employing electroluminescent, light-emitting, display elements are well known. The display elements may comprise organic thin film electroluminescent elements, using, for example, organic materials, or other light emitting diodes (LEDs), using traditional III-V semiconductor compounds.
Recent developments in organic electroluminescent materials, particularly polymer materials, have demonstrated their ability to be used practically for video display devices. These materials typically comprise one or more layers of an electroluminescent material, for example a semiconducting conjugated polymer, sandwiched between a pair of electrodes, one of which is transparent and the other is of a material suitable for injecting holes or electrons into the polymer layer. The polymer material can be fabricated using a chemical-vapour deposition (CVD) process, or simply by a spin coating technique using a solution of a soluble conjugated polymer.
The invention is concerned with active matrix display devices. In active matrix displays, addressing takes place completely behind the-electroluminescent elements. The front surface of the display is coated with a continuous electrode while the rear surface electrode is patterned into individual pixels and their pixel circuits. Thin film transistors (TFTs) act as switches and (current) driving elements for each pixel.
The switching TFT is addressed by a set of narrow multiplexed electrodes (gate lines and power lines) running along the gaps between pixels. A pixel is addressed by applying a voltage to a gate line that switches the TFT on and allows charge from the power line to flow on to the pixel circuit electrode, for example the gate of the drive TFT. This sets up a current through the pixel and turns it on. The brightness of the pixel is determined during the addressing, by the current generated by the drive TFT. An image is created as the addressing circuitry scans across the matrix. A picture frame thereby incorporates addressing, during which the brightness of the pixel is dictated, and a duty cycle, during which the pixel is driven by the power line and light is emitted. The length of a duty cycle is the ratio between the length of time the power line drives the pixel and the total length of the picture frame. Of course, the length of the picture frame depends directly on the update frequency of the display. The duty cycle can be given as a percent ratio, for example a 50% duty cycle means that the pixel is driven for half the time of each picture frame. Of course, it can also be given as an absolute value, i.e. the time length of the cycle. In the following, a long duty cycle refers to driving the pixel for a large part of the frame, whereas a short duty cycle refers to driving the pixel for a small part of the frame.
Examples of an active matrix electroluminescent display are described in EP-A-0653741 and EP-A-0717446. Unlike matrix liquid crystal display devices, in which the display elements are capacitive and therefore take virtually no current and allow a drive signal voltage to be stored on the capacitance for the whole frame period, the electroluminescent display elements need to continuously pass current to generate light.
A problem with known organic electroluminescent materials, particularly polymer materials, is that they exhibit poor stability and suffer ageing effects whereby the light output for a given drive current is reduced over a period of time of operation. While in certain applications such ageing effects may not be critical, the consequences in a pixelated display can be serious, as a viewer can easily perceive any slight variation in light output from pixels.
The lifetime problem for PLED has been treated in a multiplicity of patent applications, see for example U.S. Pat. No. 6,144,162 and WO 01/26087 A1, where the lifetime of the pixels is extended by compensating the degradation using an increased driving current during the duty cycle. However, those solutions do not really solve the underlying problem, which is the degradation of the pixels. They merely alleviate the symptoms of the problem, by adjusting the driving current.